I disagree with 500grains. Three firearms between two guys is fine. Three firearms for one person is a hassle as he says.

Split your ammo in two for each calibre and have each person carry half. As long as it is legal for you to do this if one guys bag goes missing you still have ammo to use. If you need to split the rifles, pack the .375 and .500 in separate cases and each hunter has one, perhaps the .338 with the .500. Note have a look at licence costs etc as splitting ammo and rifles may cost more in Tanz, I'm not sure.

I like soft points for buffalo so would take a mixture of softs and FMJs for both the .500 and .375 for the buffalo. But mostly SPs for the .375. All SPs for the .338.

Double check by weighing it. If too heavy and I doubt it, reduce the .500 ammo a bit. You won't use 30 .500's unless you do a lot of shooting or shoot targets.

Ndumo's answer looks pretty good. It depends though on what you want to use. The .500 just for buffalo and then the other rifles, or try out the .500 on plains game as well. A tracker will be able to carry it with you for when needed.

It looks like you will have a fun trip so good luck and post photos and stories when you return.